The invention relates to a voltage transformer circuit for a spark plug and a method for ionic current measurement.
An arc discharge is created between the electrodes of a spark plug by applying a high voltage, in order to ignite fuel in the combustion chamber of an engine. If no arc discharge is burning, it is possible, by applying a somewhat lower voltage, to measure the electrical conductance of the gas mixture between the electrodes of the spark plug. An ionic current measurement of this type makes it possible to obtain valuable information about the state of the combustion chamber of the engine. Ignition systems which allow ionic current measurements are for example known from U.S. Pat. No. 5,866,808 and DE 39 34 310 A1.
Ignition systems of this type consist of a spark plug and a voltage transformer circuit, which generates a secondary voltage of more than 10 kV from a primary voltage, generally the vehicle system voltage of a vehicle, by means of a transformer. When the primary voltage is applied to the transformer, a strong magnetic field change immediately occurs on the secondary side thereof, so that a voltage is induced in the electrodes of the spark plug. Under certain circumstances, this may already lead to an arc discharge and thus effect an undesired premature ignition of the mixture in the combustion chamber of the engine. To prevent this, modern ignition systems are often equipped with a switch-on spark suppression diode, which is arranged between the spark plug and the transformer. The switch-on spark suppression diode blocks a secondary current, which is caused by switching on the ignition system, that is to say the application of the primary voltage, and therefore prevents a premature voltage increase at the electrodes of the spark plug.
Ignition systems which contain a switch-on spark suppression diode and allow ionic current measurement are problematic in that the vehicle system voltage of a vehicle is not sufficient for ionic current measurement because of the relatively high electrical resistance of the combustion chamber contents. Rather, a higher voltage is required, typically about 100 V or more. In ignition systems without a switch-on spark suppression diode, the voltage for ionic current measurement is delivered by a capacitor, which is charged by the secondary voltage of the voltage transformer circuit, whilst the secondary voltage is applied at the spark plug. A corresponding circuit is illustrated in FIG. 1. After the arc discharge is extinguished, the capacitor is discharged by means of an ionic current, which flows between the electrodes of the spark plug. Using this circuit, the ionic current can be measured as a voltage drop at the resistor. The discharge current of the capacitor of such a circuit flows in the opposite direction through the spark plug to the charging current of the same, which effects an arc discharge of the spark plug. A switch-on spark suppression diode would therefore block the discharge current of the capacitor and therefore also of the ionic current.
It is inherently possible to generate the voltage for an ionic current measurement from the vehicle system voltage by means of a further voltage transformer. However, this possibility is very complex and it has therefore not been possible for it to gain popularity.